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Is there a cipher that well suited for preschool-aged children to work out on pencil and paper, to learn concepts of block chains?

It must have the following requirements:

  1. Able to be easily verified
    a. For example, Joe submitted red, but Suzy submitted blue. Everyone knows Suzy is right...somehow. (Color-specific results are not required but used to illustrate that the cipher need not be number-based, although that is fine too)
  2. Able to be easily ciphered with pencil and paper, or drawings, or in a child's head
  3. Able to use a previous result (minus the genesis of course), to create a 'blockchain' of sorts

The cipher may be low-complexity. It does not matter if it is easily broken.

Most ciphers are geared towards being easy to verify, but difficult to decipher. This situation is a bit different in that it does not have to be difficult to decipher. Out of the box ideas are quite welcome.

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    $\begingroup$ I daresay your biggest problem is going to be motivating this. A vast majority of well-educated adults don't care enough to understand the purpose, let alone the mechanism, of blockchain. Ciphers OTOH could be fun. (But before you can appreciate even them you need theory of mind, which only kicks in around 4-5 years old, as well as a firm grasp of the alphabet and reading.) $\endgroup$ – j_random_hacker Mar 7 '18 at 5:48
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    $\begingroup$ I think you want to be on Computer Science Educators. Should we migrate the question? $\endgroup$ – Raphael Mar 7 '18 at 11:22
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I'd suggest you don't try to create or use a cipher. Rather, I think maybe a more interesting way to view blockchain systems is as a log that is created through a distributed lottery.

So, in lesson one, you could describe how to keep track of something (account balances? something more relevant?) via a sequence of log entries written on the board. This would be a centralized scheme.

In lesson two, you can make it distributed. In each stage, have a lottery where you randomly pick one of the students to win the lottery. The winner of the lottery gets to write down the next log entry on the board. Everyone else watching can see whether that student did it correctly and fairly. If not, the next winner ignores the bogus stuff written on the board.

No cryptography is needed. You just need a way to randomly pick one student in the room, in a fair way. This could be done with numbered balls in a box, or any number of other ways.

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Would a clock face fit the bill? The position of the hands on the clock is a "block", and you generate a new block in the chain by adding (optionally subtracting) some amount of time to it.

E.g.:

e(x, prev) = (x + prev) % 12:00

  1. Suzy starts things with her "6": e(6, 0:00) = 6:00
  2. Joe correctly submits "3": e(3, 6:00) = 9:00
  3. Suzy correctly submits "5": e(5, 9:00) = 2:00
  4. Everyone can tell that Joe isn't right when he says he submitted "1" and got 5:00 instead of 3:00

It's quite easy to reverse this "cipher", but is very simple and visual. You can also readily check if Suzy or Joe correctly submitted the right thing.

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